Time-of-flight mass spectrometers for improving resolution and mass range employing an impulse extraction ion source

ABSTRACT

A miniature time-of-flight mass spectrometer (TOF-MS) and method for increasing the collection efficiency of laser-desorbed ions in a miniature time-of-flight mass spectrometer (TOF-MS) is provided. The method provides a laser pulse generated by an ionization extraction device within the TOF-MS; maintains a sample plate potential at a ground level for a fixed delay period of about 50 ns; and uses a high voltage switch to sharply increase the sample plate potential up to 10 kV/mm.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of prior filed co-pendingU.S. Provisional Patent Application No. 60/396,896, filed Jul. 17, 2002,the contents of which are incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention generally relates to miniaturetime-of-flight mass spectrometers (TOF-MS) for improving resolution andmass range employing an impulse extraction ion source and methods forits use.

[0004] 2. Description of the Related Art

[0005] Miniature time-of-flight mass spectrometers (TOF-MS) have thepotential to be used in numerous field-portable and remote samplingapplications due to their inherent simplicity and potential forruggedization. However, these miniature time-of-flight massspectrometers traditionally suffer from low mass resolution due to thereduced drift length of the analyzer. Attempts have been made to recoverthis reduction in mass resolution, by using, for example, ionreflectors. Typically, though, ion reflectors exhibit a limited usefulmass range, typically less than 5 kDa. While this range is useful formany biological agents, a much wider mass range is required to detectintact proteins and numerous biological warfare agents. To this end,several forms of “pulsed extraction (PE)” or “delayed extraction”techniques have been developed to reduce the peak widths of ionsdetected in the TOF analyzer. These methods, however, improve theresolution for only a fraction of the total mass spectrum for any givendelay setting thereby requiring the TOF analyzer to be scanned in orderto achieve good resolution across a broad mass range.

[0006] A need therefore exists for an apparatus and method that does notrequire the TOF analyzer to be scanned in order to achieve goodresolution across a broad mass range.

SUMMARY OF THE INVENTION

[0007] In accordance with the present invention, a device for increasingthe mass resolution of laser-desorbed MALDI ions across a wide massrange is provided, the device comprising:

[0008] an ionization extraction device having an unobstructed centralchamber for guiding ions there through;

[0009] a microchannel plate detector assembly having channel extendingthrough at least a portion of the assembly;

[0010] a flexible circuit-board reflector, wherein said channel isaligned with a central axis of said ionization extraction device and acentral axis of said reflector; and a voltage switch for increasing asample plate potential sharply.

[0011] Further in accordance with the present invention is a method forincreasing the mass resolution of laser-desorbed ions across a wide massrange in a miniature time-of-flight mass spectrometer (TOF-MS), saidions being desorbed through matrix assisted laser desorption/ionization(MALDI), said method comprising the steps of:

[0012] providing a laser pulse for ion creation in a source region;

[0013] maintaining a sample plate potential at a ground level for adelay period; and

[0014] increasing said sample plate potential sharply after the delayperiod.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The above and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

[0016]FIG. 1A is a cross-sectional view of an example gridless, focusingionization extraction device for a TOF-MS according to the presentinvention;

[0017]FIG. 1B is a potential energy plot of the electric field generatedby the gridless, focusing ionization extraction device;

[0018]FIG. 2A is a perspective view of an example flexible circuit-boardreflector in a rolled form according to the present invention;

[0019]FIG. 2B is top view of the flexible circuit-board reflector in anunrolled form;

[0020]FIG. 3A is a perspective view of an example center-holemicrochannel plate detector assembly according to the present invention;

[0021]FIG. 3B is a cross-sectional, exploded view of the center-holemicrochannel plate detector assembly showing the internal components;

[0022]FIGS. 4A and 4B illustrate the detector response waveform for boththe single ion signal from a conventional disk anode detector assemblyand the center-hole microchannel plate detector assembly having a pinanode, respectively;

[0023]FIG. 5 is a cut-away view of the TOF-MS having the gridless,focusing ionization extraction device, the flexible circuit-boardreflector and the center-hole microchannel plate detector assemblyaccording to the present invention; and

[0024]FIG. 6 is a block diagram depicting the mass spectrometeraccording to an embodiment of the present invention;

[0025]FIG. 7 is a pulse sequence used for the impulse extractiontechnique; and

[0026]FIG. 8 is an impulse delay spectra for Cytochrome C and BovineSerum Albunium (BSA) showing several proteins covering a broad massrange.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The present invention is a new type of delayed extraction ionsource with at least two novel features distinguishing it fromtraditional PE techniques. The source region employed is significantlyshorter than conventional pulsed extraction sources and the delay timebefore application of the extraction pulse is significantly reduced. Theresult is a significant reduction in peak width for high mass ionscompared to the continuous extraction mode in a linear time-of-flightinstrument. Although the ion peak widths are found to be somewhatbroader than those acquired using traditional PE methods, the newimpulse technique is found to be effective over a very wide mass rangerequiring no scanning of the instrument's voltage or time delaysettings.

[0028] One example of the gridless, focusing ion source that can be usedfor the inventive technique has been described in a prior filedco-pending U.S. patent application Ser. No. 10/220,865, filed Sep. 6,2002, the contents of which are included herein by reference. Theminiature time-of-flight mass spectrometer (TOF-MS) of the gridless,focusing ion source of the above referenced application include:

[0029] A. The gridless, focusing ionization extraction device,

[0030] B. The flexible, circuit-board reflector, and

[0031] C. The center-hole microchannel plate detector assembly.

[0032] Each of these features of the copending application will bedescribed below for a better understanding of the present invention.

[0033] A. Gridless, Focusing Ionization Extraction Device

[0034] The gridless, focusing ionization extraction device increases thecollection efficiency of laser-desorbed ions from a surface. Theionization extraction device is shown by FIG. 1A and designatedgenerally by reference numeral 100. The device 100 has a preferredlength of approximately 17-25 mm and includes a series of closely spacedmicro-cylinders 110 a-c mounted within an unobstructed central chamber105 which is defined by the housing 115. The housing is constructed fromone or more insulating materials, such as ceramics, Teflon, andplastics, preferably, PEEK plastic.

[0035] The micro-cylinders 110 a-c are constructed from metallicmaterials, such as stainless steel and may have varying thicknessranges. Further, it is contemplated that each micro-cylinder isconstructed from a different metal and that each micro-cylinder has adifferent thickness. The micro-cylinders 110 create an extremely highion acceleration/extraction field (up to 10 kV/mm) in region 120, asshown by the potential energy plot depicted by FIG. 1B, between a flatsample probe 130 and an extraction micro-cylinder 110 a.

[0036] Ions are created in region 120 by laser ablation or matrixassisted laser desorption/ionization (MALDI). The ions are thenaccelerated by the ion acceleration/extraction field in region 120.

[0037] The ions are slowed in a retarding field region 150 between theextraction micro-cylinder 110 a and the middle micro-cylinder 110 b. Theretarding field region 150 serves both to collimate the ion beam, aswell as to reduce the ion velocity. The ions are then directed throughthe middle micro-cylinder 110 b, where the ions are accelerated again(up to 3 kV/mm as shown by FIG. 1B).

[0038] After traversing through the micro-cylinders 110 a-c, the ionsenter a drift region 160 within the chamber 105 where the potentialenergy is approximately 0 kV/mm as shown by the potential energy plotdepicted by FIG. 1B and referenced by numeral 160′. Reference number 170in FIG. 1B references the ion trajectories through the device 100.

[0039] The series of micro-cylinders 110 a-c minimizes losses caused byradial dispersion of ions generated during the desorption process.Although the ionization extraction device 100 of the present inventionemploys a very high extraction field 120, the ions are slowed prior toentering the drift region 160, thus resulting in longer drift times (orflight duration) and hence increased ion dispersion of the ions withinthe drift region 160.

[0040] Furthermore, the performance of the ionization extraction device100 is achieved without the use of any obstructing elements in the pathof the ions, such as grids, especially before the extractionmicro-cylinder 110 a, as in the prior art, thus eliminating transmissionlosses, signal losses due to field inhomogeneities caused by the gridwires, as well as the need for periodic grid maintenance.

[0041] B. Flexible, Circuit-Board Reflector

[0042] Ion reflectors, since their development 30 years ago, have becomea standard part in many TOF-MSs. While there have been improvements inreflector performance by modifications to the voltage gradients, themechanical fabrication is still based on stacked rings in mostlaboratory instruments. In such a design, metallic rings are stackedalong ceramic rods with insulating spacers separating each ring from thenext. While this has been proven to be satisfactory for the constructionof large reflectors, new applications of remote TOF mass analyzersrequire miniaturized components, highly ruggedized construction,lightweight materials, and the potential for mass production.

[0043] To this end, the ion reflector of the present invention shown byFIGS. 2A and 2B and designated generally by reference numeral 200 wasdeveloped utilizing the precision of printed circuit-board technologyand the physical versatility of thin, flexible substrates. A series ofthin copper traces (e.g., 0.203 mm wide by 0.025 mm thick) 210 areetched onto a flat, flexible circuit-board substrate 220 having tabs 225protruding from two opposite ends (FIG. 2B). The circuit-board substrate220 is then rolled into a tube 230 (FIG. 2A) to form the reflector body,with the copper traces 210 facing inward, forming the isolated ringsthat define the voltage gradient.

[0044] The thickness and spacing of the copper traces 210 can bemodified by simply changing the conductor pattern on the substrate sheet220 during the etching process. This feature is particularly useful forthe production of precisely tuned non-linear voltage gradients, whichare essential to parabolic or curved-field reflectors. The trace patternon the circuit-board substrate 220 shown in FIGS. 2A and 2B represents aprecision gradient in the spacing of the traces 210. Thus, in theresultant reflector, a curved potential gradient is generated byemploying resistors of equal value for the voltage divider network.

[0045] In an embodiemtn, the reflector is constructed from acircuit-board with equally-spaced copper traces 210 used in conjunctionwith a series of potentiometers to establish a curved potentialgradient.

[0046] Once etched, the circuit-board substrate 220 is rolled around amandrel (not shown) to form a tubular shape as shown in FIG. 2A. Fivelayers of fiberglass sheets, each approximately 0.25 mm thick, are thenwrapped around the circuit-board substrate 220. The length of thecurving edge of the board 220 is approximately equal to thecircumference of the mandrel. When the sheets are wrapped around therolled circuit-board, a slight opening remains through which a connectorend 240 of the inner circuit-board can extend. The position of eachsuccessive sheet is offset slightly with respect to the previous sheetso that a gradual “ramp” is formed, thereby guiding the flexiblecircuit-board substrate 220 away from the mandrel.

[0047] The reflector assembly is heated under pressure at 150.degree. C.for approximately two hours, followed by removal of the mandrel. Wallthickness of the finished rolled reflector assembly is approximately 1.5mm. A multi-pin (preferably, 50-pin) ribbon-cable connector 250 issoldered onto a protruding circuit-board tab 260 so that a voltagedivider resistor network can be attached to the reflector. Alternately,soldering pads for surface-mount resistors can be designed into thecircuit-board layout, allowing the incorporation of the voltage dividernetwork directly onto the reflector assembly.

[0048] Finally, polycarbonate end cap plugs (not shown) are fitted intothe ends of the rolled reflector tube 230 to support the assembly aswell as provide a surface for affixing terminal grids. Vacuum testsindicate that the circuit-board and fiberglass assembly is capable ofachieving vacuum levels in the low 10.sup.-7 torr range.

[0049] The reflector 200 is disclosed in a U.S. Provisional PatentApplication Serial No. 60/149,103, filed on Aug. 16, 1999, andincorporated herein its entirety by reference.

[0050] C. Center-Hole Microchannel Plate Detector Assembly

[0051] For miniature TOF mass spectrometers, the center hole (coaxial)geometry is a highly desirable configuration because it enables thesimplification of the overall design and allows for the most compactanalyzer. However, the poor signal output characteristics ofconventional center hole microchannel plate detector assemblies,particularly the problem with signal “ringing”, clutter the baselineand, as a consequence, adversely affects the dynamic range of theinstrument. This limitation severely reduces the chance of realizinghigh performance in miniature TOF instruments, since low intensityfragment or product ion peaks can be obscured by baseline noise.Improvements to the analog signal quality of center-hole channel-platedetectors would therefore increase the ultimate performance of the massspectrometer, particularly the dynamic range.

[0052] Commercially available coaxial channel-plate detectors rely upona disk-shaped center-hole anode to collect the pulse of electronsgenerated by the microchannel plates. The anode is normally matched tothe diameter of the channel-plates, thereby, in theory, maximizing theelectron collection efficiency. However, the center-hole anode createsan extraneous capacitance within the grounded mounting enclosure. Thecenter-hole anode also produces a significant impedance mismatch whenconnected to a 50 .OMEGA. signal cable. The resultant ringing degradesand complicates the time-of-flight spectrum by adding a high frequencycomponent to the baseline signal. Moreover, the disk-shaped anode actsas an antenna for collecting stray high frequencies from the surroundingenvironment, such as those generated by turbo-molecular pumpcontrollers.

[0053] The pin anode design of the center-hole microchannel platedetector assembly of the present invention as shown by FIGS. 3A and 3Band designated generally by reference numeral 300 has been found tosubstantially improve the overall performance of the detector assembly300. For enhanced sensitivity, the assembly 300 includes a clamping ring305 having an entrance grid 310 which is held at ground potential whilea front surface 315 of a center-hole microchannel plate assembly 320(FIG. 3B) is set to −5 kV, post-accelerating ions to 5 keV. The clampingring 305 is bolted to an inner ring 325. The inner ring 325 is bolted toa spherical drum 330 having a tube 332 extending from a center thereofand a shield 334 encircling an outer surface 336. The tube 332 defines achannel 338. The shield is fabricated from any type of conductingmaterial, such as aluminum, and stainless steel foil.

[0054] Using voltage divider resistors, the rear of the plate assembly320 is held at 3 kV as shown by FIG. 3B. Since the collection pin anode350 is isolated from the center of the detector assembly 300, i.e.,isolated from the channel 338 defined by the tube 332, its potential isdefined by an associated detector amplifier (nominally ground). Thus,electrons emitted from a rear microchannel plate 355 of the plateassembly 320 will be accelerated toward the grounded anode 350regardless of the anode's size, geometry, or location. The pin anode 350is located about 5 mm behind the rear microchannel plate 355.

[0055] It has been demonstrated that the pin anode 350 significantlyimproves the overall performance of the detector assembly 300. Theinventive center-hole microchannel plate detector assembly 300 virtuallyeliminates the impedance mismatch between the 50 ohm signal cable andthe electron collection surface, i.e., the pin anode 350.

[0056]FIGS. 4A and 4B compare the single ion detector response for boththe conventional disk anode and the pin anode configurations. It isevident from FIGS. 4A and 4B that ringing is significantly reduced andthe ion pulse width is reduced to a value of 500 ps/pulse, limited bythe analog bandwidth of the oscilloscope used for the measurement (1.5GHz: 8 Gsamples/sec), when using the pin anode configuration of thepresent invention. Furthermore, the background signal in thetime-of-flight data caused by spurious noise is found to be much quieterwhen the pin anode configuration is used.

[0057]FIG. 5 depicts a TOF-MS designated generally by reference numeral500 containing the focusing ionization extraction device 100, theflexible circuit-board reflector 200, and the microchannel platedetector assembly 300. The overall length of the entire TOF-MS isapproximately 25 cm. A laser 510, such as a nitrogen laser, is used foracquiring MALDI and laser ablation spectra. The laser 510 emits a laserbeam 520 which is directed through the TOF-MS 500 using two mirrors 530a, 530 b. The TOF-MS 500 is enclosed within a vacuum chamber 525 andmounted into position by a bracket/rod assembly 535 such that the laserbeam 520 passes through a central path defined by the inventivecomponents. In an experimental study, time-of-flight data was acquiredon a LeCroy 9384 Digital Oscilloscope (1 GHz: 2 Gsam/s) used inconjunction with spectrum acquisition software.

[0058] The gridless, focusing ion source discussed above was used forthe application of a constant field, whereas in the present invention,the gridless, focusing ion source is used in a pulsed extraction mode.FIG. 6 is a block diagram depicting the mass spectrometer of the presentinvention. FIG. 7 shows the pulse sequence used for the impulseextraction technique. The sample plate 130 is held at ground potential(10) for approximately 50 ns following the laser ionization pulse (12).After this fixed delay time, plate 130 is pulsed up to +10 kV (14) usinga high voltage delay generator 602. In this case, a high voltage switch601 (Behlke) is employed to supply the fast rise-time high voltagepulse. The high voltage delay generator 602 and high voltage switch 601are under control of controller 600.

[0059]FIG. 8 illustrates the impulse extraction spectra for Cytochrome C(16) and Bovine Serum Albunium (18). Both spectra (16) and (18),acquired on a 5″ linear TOF-MS, show several analytes covering a broadmass range. The pulse delay time, used to generate these data, was 50 nsthe same for both analyses. The improved resolution of impulseextraction over constant field extraction is possibly explained by thefact that it appears large ions formed by the matrix assisted laserdesorption/ionization (MALDI) process require 10's of nanoseconds toacquire a charge. Allowing the desorbed molecules to drift in a fieldfree region for at least 50 ns enables charge transfer to occur throughmultiple collisions with the matrix ions, yet they have insufficienttime to expand far into the source region, a migration of less than 10μm.

[0060] Resultant peak widths of impulse extraction are thereforesignificantly sharper since ions of a given m/z are acceleratedsimultaneously while at the same position in the source region and henceacquire the same velocity. In contrast, under constant field extractionconditions, the ions, as soon as they become charged, are acceleratedover an extended period of time. Moreover, this process occursthroughout a range of acceleration potentials due to the field gradientin the source region. The longer the time required for ionization tooccur, the broader the resultant TOF peaks will be

[0061] What has been described herein is merely illustrative of theapplication of the principles of the present invention. For example, thefunctions described above and implemented as the best mode for operatingthe present invention are for illustration purposes only. Otherarrangements and methods may be implemented by those skilled in the artwithout departing from the scope and spirit of this invention.

What is claimed is:
 1. A method for increasing the mass resolution oflaser-desorbed ions across a wide mass range in a miniaturetime-of-flight mass spectrometer (TOF-MS), said ions being desorbedthrough matrix assisted laser desorption/ionization (MALDI), said methodcomprising the steps of: providing a laser pulse for ion creation in asource region; maintaining a sample plate potential at a ground levelfor a delay period; and increasing said sample plate potential sharplyafter the delay period.
 2. The method of claim 1, wherein said delayperiod is about 50 ns.
 3. The method of claim 1, wherein said increasingstep is performed by a high voltage switch, said sample plate potentialis increased up to 10 kV/mm.
 4. A device for increasing the massresolution of laser-desorbed MALDI ions across a wide mass range, saiddevice comprising: an ionization extraction device having anunobstructed central chamber for guiding ions therethrough; amicrochannel plate detector assembly having a channel extending throughat least a portion of the assembly; a flexible circuit-board reflector,wherein said channel is aligned with a central axis of said ionizationextraction device and a central axis of said reflector; and a voltageswitch for increasing a sample plate potential sharply.
 5. The device ofclaim 4, wherein the ionization extraction device includes: a firstregion for creating an ion acceleration/extraction field measuring up to10 kV/mm and for accelerating the ions accelerating ions; a secondregion for de-accelerating the ions to collimate the ions and to reducethe velocity of the ions; and a third region for causing the ions todisperse and having an electric field measurement of approximately 0kV/mm.
 6. The device of claim 4, further comprising a laser forproviding a laser pulse for ion creation in a source region.
 7. Thedevice of claim 6, wherein said voltage switch increases said sampleplate potential up to 10 kV/mm after a delay period.
 8. The device ofclaim 7, wherein said delay period is about 50 ns.
 9. The device ofclaim 7, wherein said sample plate potential is increased up to 10kV/mm.
 10. An ionization extraction device for use in a TOF-MScomprising: a housing defining an unobstructed central chamber forguiding ions therethrough; a first region within the central chamber foraccelerating ions for creating an ion acceleration/extraction fieldmeasuring up to 10 kV/mm for accelerating the ions; a second regionwithin the central chamber in proximity to the first region forde-accelerating the ions entering therein; a third region within thecentral chamber for causing the ions to disperse and having an electricfield measurement of about 0 kV/mm; a laser for providing a laser pulsefor ion creation in a source region; and a voltage switch for increasinga sample plate potential sharply up to 10 kV/mm after a delay period ofabout 50 ns.
 11. A method for increasing the mass resolution oflaser-desorbed ions in a TOF-MS, said method comprising the steps of:providing an ionization extraction device within the TOF-MS, theionization extraction device having an unobstructed central chamberhaving a first region and a second region; creating an ionacceleration/extraction field measuring up to 10 kV/mm within the firstregion; creating ions in the first region by one of laser ablation andmatrix assisted laser desorption/ionization (MALDI); aligning a centralaxis of the ionization extraction device with a tubular channel of amicrochannel plate detector assembly of the TOF-MS; aligning a centralaxis of the ionization extraction device with a central axis of acircuit-board reflector of the TOF-MS; providing a laser pulse for ioncreation in a source region; maintaining a sample plate potential at aground level for a delay period; and increasing said sample platepotential sharply after the delay period.
 12. An apparatus for improvingresolution and mass range for a time-of-flight mass spectrometer havinga sample plate, comprising: a high voltage delay generator for providinga high voltage pulse; a switch connected to the sample plate, the highvoltage delay generator and ground; and a controller for controllingsaid switch and high voltage delay generator, wherein the sample plateremains connected to ground for a predetermined time period after saidplate is pulsed with a laser ionization pulse, and after thepredetermined time period the sample plate is supplied with the highvoltage pulse.